ABAQUS; Weird U3 results 1

[Entherm]

In my ABAQUS, I have two beams and have applied axial compression to one of the beams. Both the beams have the same non-linear material property but different geometric properties. The other beam interacts with the first beam (the one in which the load was applied) by the following interaction. (firs beam nodes are master)

The U1 is not constrained to prevent the transfer of axial force from the first beam to the second. I couldn't understand why I am getting these weird U3 values. Why does it decrease from 0.5 sec? I am assuming this is the point where the first beam yields, but why would the U3 decrease?. Any insights would be highly appreciated.

 

[FEA way]

What is the actual purpose of this analysis ? Why are the beams connected this way ? Maybe you should use different type of constraint.

 

[Entherm]

The purpose is to apply the compressive force greater than the buckling capacity of the first beam. The second beam is used such that due to the constraint applied, the stiffness of the second beam engages and prevents buckling. The U1 is not constrained because we don't want the beams to interact axially i.e. no transfer of axial force between them. The buckling is in fact prevented but with these results.